In prior art vehicle transmissions such as transmission 10 depicted in FIG. 1 which is typical of a THM 700 R4 (General Motors) transmission (or its replacement, 4L 60E), there is a 2-4 band 12 which is applied in both second gear and fourth gear (overdrive). Band 12 wraps about a reverse input drum or member 14 (only partially shown) and has one end anchored to a case 16 of transmission 10 by a band anchor pin 18. As known, input drum 14 is tanged to a reaction sun shell (not shown) which is splined to the reaction sun gear (not shown). Thus, when the a 2-4 band 12 is applied by the 2-4 servo assembly 20, reverse input drum 14, the reaction sun shell, and the reaction sun gear are all held stationary.
Servo assembly 20 applies band 12 in both second and fourth gears, and is used as an accumulator in third gear. In second gear, 2nd clutch fluid acts on a 2nd apply piston 22 providing sealed movement between case 16 and an inner piston housing 24. This forces a longitudinal 2-4 apply pin 26, which has a proximal end 28 in engagement with a receiving stop 30 on band 12, to move axially and hence to compress 2-4 band 12 and stop reverse input drum 14 from rotation. The apply rate or force of 2-4 band 12 in second gear is mainly controlled by the compressing of a servo cushion spring 32 and the 1-2 accumulator system working against a return spring 34. Cushion spring 32 is held in place between the cupped end of 2nd apply piston 22 and a groove provided in cushion spring retainer 36, with cushion spring retainer 36 being held movably inside of the cupped end of 2nd apply piston 22 by retainer ring 38 as shown.
In third gear, 2-4 servo assembly 20 releases the force of apply pin 26 against stop 30 by channeling 3rd accumulator fluid against cushion spring retainer 36 causing and hence to counteract 2nd clutch fluid pressure against 2nd apply piston 22. This 3rd accumulator fluid also seats an accumulator exhaust checkball as known (not shown). The force of 3rd accumulator fluid and of cushion spring 32 overcomes 2nd clutch fluid pressure. Thus, 2nd apply piston 22 and apply pin 26 move away from band 12 to release band 12 from the reverse input drum 14. 3rd accumulator fluid acting on 2nd apply piston 22 also acts as an accumulator to help control the apply rate of the 3-4 clutch as known (not shown).
In fourth gear, 4th clutch fluid is fed through the hollow distal end of apply pin 26 and out of the end thereof against 4th apply piston 40. 4th apply piston 40 is mounted inside of a 2-4 cover or housing 42. Housing 42 is held in place in case 16 against inner piston housing 24 by a retaining ring 44, and is sealed to case 16 using a seal 45. Apply pin 26 is connected to 4th apply piston 40 by an apply pin spring 46 compressed between 2nd apply piston 22 and a washer 50 held on apply pin 22 by a snap ring 52. This added pressure force from the 4th clutch fluid, in addition to the 2nd clutch fluid pressure still present on 2nd apply piston 22, overcomes the pressure from cushion spring 32 and the 3rd accumulator fluid. This moves 4th apply piston 40 and 2-4 apply pin 26 toward band 12 to apply band 12 and obtain fourth gear.
While servo assembly 20 performs satisfactory when new, over time the transmission loosens, hydraulic pressures decrease as pumps are less efficient, and seals leak. Consequently, such transmissions are prone to sluggish 3rd to 4th gear upshifts.